If the restriction of OS X to Apple hardware uses code embedded in the hardware (which Apple makes), then the kernel source may be as open as can be. One key in a cryptographic pair could be in the hardware, say, and the other could be in the kernel source. Running OS X on a non-Apple PC might require a mod chip like people have done with the XBox. Plus, as this article indicates, not all of OS X is open source now anyway, so whatever interesting parts of OS X handle the Apple hardware restriction may just be in a closed-source portion, with some bare hooks into the kernel.